Since the inception of signal transmission, there has been a struggle to reduce the amount of interference in the received signal. Interference can be caused by a number of factors. For example, various environmental factors (e.g., mountains, buildings, other man-made structures, etc.) can contribute to signal interference when they are located within the transmission path between a transmitter and a receiver.
Also, with the proliferation of many different methods and devices to transmit signals, there has been a significant increase in the amount of interference that can occur when signals interfere with each other. This can occur when signals propagate through the same medium (e.g., airspace, cable, wire, etc.) as other signals.
Various methods and techniques have been employed to minimize signal interference. For example, the receiving device may include various types of filters to remove unwanted interference from the received signal. These filters have been known to include various types of band pass filters (e.g., filters to permit signals within a predetermined frequency range to pass but removing all other portions of the signal) alone or in combination.
FIG. 1 shows one example of a switched filter bank, which has been used to remove unwanted interference from a received signal. Switched filter bank 100 includes input 102 to receive a signal to be filtered and output 104 to output the filtered signal. A controller (not shown) controls switches 106, 108, and 110 between an open position and a closed position. In the closed position of the switch, the signal can move through the switch (e.g., switch 106a) and its respective filter (e.g., filter 112). In the open position, the signal may not pass through the switch or its respective filter.
For example, if switches 106a and 106b are closed, the signal can be filtered by filter 112. Likewise, if switches 108a and 108b are closed, the signal can be filtered by filter 114, and if switches 110a and 110b are closed, the signal can be filtered by filter 116. Depending on the desired frequency response, one or more of filters 112, 114, and 116 can be used to filter the signal before it is output via output 104.
However, there are several drawbacks to using the switched filter bank. Specifically, the switches in the switched filter bank increase the insertion loss of the device, which degrades the sensitivity of the device. In addition, the switches in the switched filter bank increase the size and cost of the device.
Turning now to FIG. 2, a multiplexed filter is shown that has also been used to filter received signals. Multiplexed filter 200 includes input 202 and output 204. Filters 206, 208, and 210 are coupled in parallel to input 202 and output 204.
In operation, multiplexed filter 200 utilizes filters 206, 208, and 210 to pass all the desired channels (e.g., frequencies), similar to the switched filter bank, discussed above. However, multiplexed filter 200 uses input and output common node matching networks rather than switches. Although this feature reduces the insertion loss of multiplexed filter 200 when compared to the insertion loss of the switched filter bank, it also provides less selectivity than the switched filter bank since the multiplexed filter passes all frequencies in its combined pass band simultaneously. Thus, there is little interference rejection provided by the multiplexed filter.
Although not shown in the figures, tunable band pass filters have also been used, if they can meet the required performance specifications of a particular application, to filter out unwanted interference in a received signal. However, typical tunable band pass filters have been known to require complex tuning, alignment, and control functions.
There is therefore a need in the art for a device and method that can be used to filter out unwanted interference from a signal in a manner that provides lower insertion loss than the switched filter bank, better frequency selectivity than the multiplexed filter, and with less complex tuning, alignment, and control than the tunable filter.